Kotobuki is a girl in Konoha's class. She doesn't like Konoha's way of being nice to people. She thinks that he's faking.
Kotobuki actually met Konoha in junior high school once, and since that time, she's been watching him from behind. She knows that Konoha was happy because Miu was there, but she feels that he was actually not happy when she saw him again on high school. That's why it looks like Kotobuki doesn't like Konoha. She actually likes Konoha and feels hurt about the way Konoha acts now.
Even though she was Konoha's girlfriend in most of the story, Konoha chose Touko over her after what happened in "Romantie" book. Even though Konoha no longer chose her, she still likes him and is his best friend.
There was a time Nanase tried to avoid Konoha and even wanted to get him feel the pain in Minarai series. After she was rejected by Konoha, she tried to forget him and warned Hinosaka to not get involved with Konoha. She only wanted to be with Konoha who doesn't write novels at all, because his personality/expression changes when writing a novel. She was the one who told Konoha, who was afraid of writing novels, that it's ok to not write novels and she still be with him. After she told Hinosaka her feelings and the whole event in book 2 (but not Shoushin), her heart is lightened and tries to continue loving Konoha.